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United States Patent |
5,235,608
|
Konishi
|
August 10, 1993
|
Gas laser apparatus
Abstract
In a gas laser apparatus including a gas laser tube having a plasma tube
between a cathode electrode and an anode electrode, and a magnetic field
generator for generating a magnetic field through the central hole of said
plasma tube, the radius of the central hole is configured so as to avoid
ion bombardment by confining the ionized plasma within the magnetic flux
passing through the cathode electrode. For practical purposes the central
hole is tapered, at least near the entrance thereof, with its radius
larger than that of the magnetic flux entering the central hole, thereby
substantially eliminating sputtering by ion plasma at the entrance end and
on the inner wall of the plasma tube.
Inventors:
|
Konishi; Takayoshi (Tokyo, JP)
|
Assignee:
|
NEC Corporation (Tokyo, JP)
|
Appl. No.:
|
952109 |
Filed:
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September 28, 1992 |
Foreign Application Priority Data
Current U.S. Class: |
372/37; 372/61 |
Intern'l Class: |
H01S 003/00 |
Field of Search: |
372/37,61
|
References Cited
U.S. Patent Documents
3993965 | Nov., 1976 | Alves et al. | 372/37.
|
4912719 | May., 1990 | Kanamoto et al. | 372/61.
|
Primary Examiner: Lee; John D.
Assistant Examiner: Wise; Robert E.
Claims
I claim:
1. A gas laser apparatus including a gas laser tube having a plasma tube
between a cathode electrode and an anode electrode, and a magnetic field
generator for generating a magnetic field through the central hole of said
plasma tube, wherein
the radius of said central hole at any given point along the center line of
said central hole is greater than the radius R of magnetic flux B, defined
by
##EQU4##
where Rc is the radius of a circle circumscribing the cathode end
opposing said plasma tube, Bc is the magnetic flux at the center of said
circumcircle, and B is the magnetic flux density at said given axial
point.
2. A gas laser apparatus according to claim 1, wherein said capillary tube
is divided into a multiplicity of pieces coaxially spaced apart along
their common center line.
3. A gas laser apparatus including a gas laser tube having a plasma tube
between a cathode electrode and an anode electrode, and a magnetic field
generator for generating a magnetic field through a central hole of said
plasma tube, wherein
the radius of the entrance of said central hole facing said cathode
electrode is larger than the radius R of magnetic flux density B, defined
by
##EQU5##
where Rc is the radius of a circle circumscribing the cathode end
opposing said plasma tube, Bc is the magnetic flux density at the center
of said circle, and B is the magnetic flux density at said entrance of
said central hole.
4. A gas laser apparatus according to claim 3, wherein said plasma tube is
divided into a multiplicity of pieces coaxially spaced apart along their
common center line.
Description
FIELD OF THE INVENTION
The invention relates to a gas laser apparatus.
BACKGROUND OF THE INVENTION
A typical gas laser as shown in FIG. 1 has a pair of opposing mirrors 10
aligned to their common optical axis (which mirrors will be hereinafter
referred to as optical resonator), a gas laser tube 1 mounted in the
optical resonator and containing a gas such as Argon, and an
electromagnetic coil 5 for generating a magnetic field through the length
of the laser tube 1.
Mounted inside the gas laser tube 1 are a plasma tube 2 having a central
hole 2h, and a pair of a cathode electrode 3 and an anode electrode 4 at
the opposite sides of the plasma tube. The plasma tube 2 is often divided
into a multiplicity of smaller pieces which are coaxially spaced apart
along their common center line.
When a current is passed from the anode electrode to the cathode electrode,
gas discharge takes place, generating gaseous ions or plasma between them.
In order to confine the plasma within the central hole 2h of the plasma
tube 2, the electromagnetic coil 5 is impressed with a voltage to generate
a magnetic field oriented generally along the center line of the central
hole. By means of gas discharge, the ion plasma is continuously
stimulated, creating inverted population distribution over laser
transition levels and by means of optical resonator, induced laser
emission is amplified.
In such gas lasers, a portion of ions accelerated by the impressed electric
field between the electrodes bombard the entrance end and the inner wall
of the capillary tube, and can erode the entrance end and the wall due to
sputtering. Such sputtering, therefore, disadvantageously shorten life of
the laser tube.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to provide an improved laser tube
subject to only negligible sputtering by plasma at the entrance end and on
the wall of the laser tube and thus having improved durability of the
tube.
In one aspect of the invention, there is provided a gas laser apparatus
including a gas laser tube having a plasma tube between a cathode
electrode and an anode electrode, and a magnetic field generator for
generating a magnetic field through the central hole of said plasma tube,
characterized in that the radius of said central hole at any given point
along the center line of said central hole is greater than the radius R of
magnetic flux density B, defined by
##EQU1##
where Rc is the radius of a circle circumscribing the cathode end opposing
said plasma tube, Bc is the magnetic flux density at the center of said
circumcircle, and B is the magnetic flux density at said given axial
point.
This arrangement may reduce sputtering by the ion plasma on the entrance
and the wall of the capillary tube and provide durability of the laser
tube.
In another aspect of the invention, there is provided a gas laser apparatus
including a gas laser tube having a plasma tube between a cathode
electrode and an anode electrode, and a magnetic field generator for
generating a magnetic field through the central hole of said plasma tube,
characterized in that the radius of the entrance of said central hole
(facing said cathode electrode) is larger than the radius R of magnetic
flux, defined by
##EQU2##
where Rc is the radius of a circle circumscribing the cathode end opposing
said plasma tube, Bc is the magnetic flux density at the center of said
circumcircle, and B is the magnetic flux density at the center of said
entrance of said central hole.
The laser apparatus having this type of plasma tube which is simple in
design permits efficient elimination of plasma bombardment on the entrance
and the wall of the plasma tube.
Still another aspect of the invention, there is provided a gas laser
apparatus, wherein the plasma tube for use with either of the above
apparatuses is divided into a multiplicity of pieces coaxially spaced
apart along their common center line.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross section of a conventional gas laser.
FIG. 2 is a schematic cross section of a first gas laser tube according to
the invention.
FIG. 3 is a schematic cross section of a second gas laser tube according to
the invention.
In FIGS. 2-3, like or corresponding portions of the gas laser tubes are
numbered the same throughout the drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 2 shows a cross section of a major portion of a first gas laser tube
according to the invention. A plasma tube 2 is provided between a pair of
cathode electrode 3 and an anode electrode 4, which are arranged between a
pair of mirrors forming an optical resonator. The optical resonator may be
installed either inside or outside the laser tube. The plasma tube has a
first end 20 facing a cathode electrode 3 and a central hole 2h formed
therethrough. The cathode electrode 3 has an end opposing the first end of
the plasma tube 2 (said end referred to as cathode end). A magnetic flux
outlined by magnetic field lines 7 passing through the circumcircle of the
cathode end serves to confine ions accelerated toward the anode electrode.
The entrance of the central hole facing the cathode electrode 3 is tapered
in such a way that the radius Rd of the entrance is larger than the outer
diameter of the magnetic flux entering the entrance.
More specifically, given the radius Rc of the circle circumscribing the
cathode end, magnetic flux .PHI.c passing through the circumcircle is
given by
.PHI.c=.pi.Rc.sup.2 Bc Eq. (1)
where Bc is the magnetic flux density at the center of the circumcircle. It
should be noted that the radius R of the magnetic flux .PHI. at an
arbitrary point along the center line of the central capillary is given by
.PHI.=.pi.R.sup.2 B Eq. (2)
where B is the magnetic flux density at the center line of the arbitrary
point. By equating these two equations (1) and (2), the radius R is
obtained as follows.
##EQU3##
This equation may be interpreted as giving the radius of plasma flux
confined in the magnetic flux passing through the circumcircle for the
cathode electrode 3, since ionized particles in the plasma proceeds to the
anode electrode substantially along the magnetic field lines.
By choosing larger than R given by Equation (3) the inner radius of the
central hole 2h at any point along its center line, the magnetic field
lines passing through the cathode electrode 3 will not penetrate the wall
of the central hole of the plasma tube 2, thereby preventing sputtering by
ion bombardment at the entrance end and on the wall of the plasma tube.
However, for substantial reduction of sputtering in the plasma tube, it is
sufficient to make the radius of the central hole at the entrance larger
than the radius of the magnetic flux at the entrance, and to taper the
central hole only near the entrance, since a minor portion of the magnetic
field lines crossing middle of the wall will not cause serious sputtering
problems.
When an electric current is passed through between the cathode electrode 3
and the anode electrode 4, ionized plasma is generated between the
electrodes and, under the influence of the magnetic field, confined in the
magnetic flux passing through the electrodes, so that ions may pass
through the central hole without any or with only negligible interference
with the wall of the central hole.
The plasma tube described above in connection with FIG. 2 is a single
continuous piece. However, the plasma tube may be substituted for by a
multiplicity of shorter pieces 2a, 2b, through 2c coaxially spaced apart
along their common center line, as shown in FIG. 3. The central hole
formed in these pieces are tapered or configured so as to conform the
longitudinal cross section of a single piece central hole discussed in
relation to FIG. 2.
It is apparent that in this case also the outermost magnetic field lines
passing through the circumcircle of the cathode end will not intersect the
first end of the plasma tube, thereby preventing sputtering by the plasma
on that first end and providing durability of the laser tube. In addition,
the plasma tube tapered or configured in this manner may substantially
eliminate sputtering on the wall of the central hole.
In summary, the invention may greatly suppress sputtering on the plasma
tube by confining plasma within a given magnetic flux., said confinement
based on an analysis of geometrical interrelationship between the cathode
electrode and the plasma tube.
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